The invention relates to a device for machining a material with a pulsed laser beam. The invention further relates to a process for operating such a material-machining device.
In principle, the invention is applicable in the field of laser systems that serve for machining arbitrary materials. Accordingly, the material to be machined may be dead matter; but it may also be a question of living matter, for example tissue of a human eye.
In connection with the machining of materials, particularly those materials which are transparent in the visible spectral region, so-called femtosecond laser systems are gaining increasing importance. In this connection it is a question of laser systems is that generate a pulsed focused laser beam with pulse durations within the femtosecond range. Such femtosecond laser systems find application, for example, in laser-surgical ophthalmology, where they are ordinarily employed for the purpose of producing incisions in the corneal tissue or in other tissue regions of the human eye. An advantageous aspect of femtosecond laser systems is their suitability for generating three-dimensional incision figures that are arbitrary as such.
In the following the fundamental elementary process in connection with the machining of transparent materials by means of focused femtosecond laser radiation will be briefly elucidated. By virtue of the strong focusing of the laser beam into the material and on account of the transparency of the material in respect of the radiation, the laser power can be coupled into the interior without the transradiated material (e.g. corneal tissue) above the focal point being damaged. The process that takes place at the focal point is designated as photodisruption. At the focal point the threshold for the genesis of microplasma is exceeded by reason of the high-intensity radiation. Evaporation occurs of an extremely small sphere of material with a diameter of, for example, approximately 1 μm. As a result, a microbubble arises having a somewhat larger diameter, for example approximately 5-12 μm, which severs the surrounding material and subsequently diffuses completely into the environment. By virtue of the extremely short duration of action per laser pulse, no conduction of heat to the surrounding material is able to take place; all the effective energy as well as heat is dissipated again after the disappearance of the plasma.
In conventional femtosecond laser systems the focal point is capable of being controlled transversely and also longitudinally by means of a scanner. ‘Transversely’ in this connection means a direction in a plane that is orthogonal to the direction of beam propagation. ‘Longitudinally’, on the other hand, means a direction along the direction of propagation of the laser beam. If an appropriate number (for example, several thousand) of cavities that have arisen by virtue of plasma discharge are placed on top of one another three-dimensionally in the desired shape, the desired incision in the material arises.
The above process demands a very precise focal point with high peak intensity, in order to make the incisions with the desired accuracy. The focusability and the peak intensity are, however, sensitive parameters; comparatively slight perturbations in the path of propagation of the laser beam may already impair the spatial and temporal quality of the laser beam and hence the focusability and peak intensity thereof. During the machining of material it is therefore desirable to monitor the temporal and spatial quality of the laser beam continually (continuously or at least repeatedly at temporal intervals).
It is the object of the invention to specify a solution path in order to be able to monitor the radiation quality of the laser radiation in a laser system serving for the machining of material. Such a monitoring is intended to make it possible to interrupt the machining process or to react otherwise in the event of deterioration of the radiation quality. In particular, the monitoring is to be possible in real time, i.e. during the machining of material. In this case the radiation quality relates, in particular, to the focusability as well as the temporal progression of the pulse envelope (temporal envelope of the individual laser pulse), i.e. the brevity of the pulse duration. The focusability is crucially determined by the quality of the beam profile and of the wavefront of the laser radiation.